The present invention relates in general to multi-phase regulating transformers and in particular to a method of and a device for fine adjustment of the output voltage of such transformers. The invention is particularly suitable for use in electroplating installations.
As known, the output voltage of regulating transformers is adjustable within a certain range only, determined by structural limitations of the transformer. The adjustment is effected either manually or by means of an electromotive drive.
In many applications it is of importance that the output voltage be adjusted very accurately. In conventional transformers, however, the best achievable voltage resolution is that of one winding. That means that, if a regulating transformer has n windings at its tapping side, then the optimum resolution corresponding to the smallest obtainable output voltage increment .DELTA.U.sub.A EQU .DELTA.U.sub.A =U.sub.A max/n
In multi-phase regulating transformers, such as three-phase transformers, the three regulating transformers are conventionally arranged in such a manner that the three voltages are tapped by means of a common drive. Usually, the setting trolleys of the three individual transformers when connected in a star configuration, are coupled together by means of a common axle or in the case of a delta configuration by means of a chain drive. The coupling means are designed in such a manner as to provide for a rigid coupling between the trolleys and the common axis or chain drive, thus ensuring synchronous movement in all three phases. This rigid coupling of the phases and the synchronization of the movement of tapping means result in the aforementioned limit of voltage resolution of the multi-phase regulating transformer.
It is true that methods and devices have already been devised which provide a better resolution of output voltages adjusted by a multi-phase regulating transformer than that determined by the adjustment range of a single winding voltage.
For example an arrangement is known in which a regulating transformer m.sub.1 has two taps at its output. A voltage .DELTA.U.sub.A1 between the two taps feeds a second regulating transformer m.sub.2 which increases the resolution of the first mentioned transformer m.sub.1 proportionally to the number of its windings. The two transformers must be constructed for withstanding the full current.
In another prior-art solution, there are employed electronic or electromagnetic regulating members instead of a regulating transformer. These regulating members make it possible to achieve a very good voltage resolution.
Also known is a method which relatively improves the resolution of regulating transformers. With this method if, at a given input voltage U.sub.E there is produced the desired output voltage U.sub.A already at a small number of windings n.sub.1, then the resolution of voltage steps .DELTA.U.sub.A1 = EQU .DELTA.U.sub.A1 =U.sub.A1 /n.sub.1
If however the desired output voltage U.sub.A is achievable at a larger number of windings n.sub.1 only, then the resolution in comparison with windings n.sub.1 is correspondingly larger (n.sub.2 &gt;n.sub.1). Consequently, the regulation should take place in the end range of the transformer winding n. In practice, there are implied correspondingly accurately dimensioned and/or switchable transformers in series before or after the windings of the regulating transformer.
All these prior-art devices or methods, however, are possessed of the large disadvantage that they are complicated in structure and expensive. In addition, the construction of these known devices is not so strong and problem-free as in the construction of simpler multi-phase regulating transformers.